Dispersion is the splitting up of white light into its individual wavelengths, what we see as colors. Dispersion occurs when light travels through a material with transparent surfaces that are not parallel to each other, such as gemstone facets or a prism.
All the individual wavelengths are refracted differently inside a material. Red light has a smaller refractive index than blue light, thus the blue part of white light will bend more. These values are different for all gemstones, dependent upon the stone's optical density (how fast light can travel inside the gemstone).
In the chapter on refraction we learned that we measure the refractive index of a gemstone with yellow light. We could also measure it with other colors of light and as can be expected, the refractive index will be different. In gemology the value for the dispersion is the interval (difference) between the refractive index of a material measured with red light and the refractive index measured with blue light.
All transparent gemstones will show dispersion, but the dispersion colors may be masked by the body color of the gemstone. In diamonds, the color dispersion of white light is one of the causes of the spectacular "fire" in well-cut brilliants that possess good white color. This "fire" is an interaction between color dispersion and total internal reflection (the latter will be discussed later).
The refraction index of diamond (measured with yellow light) gives a refractive index of 2.417. The value for red light is 2.407 and for blue light it is 2.451. The interval between the red and the blue is: 2.407 - 2.451 = 0.044. Thus, the dispersion value of diamond is 0.044.
This example shows that decreasing (shorter) wavelengths, as in blue light, give increasing indices of refraction.
The red light we use for this purpose travels at a wavelength of 686.7nm and the blue light at 430.8nm. The interval between the refractive indices of the red and the blue gives the dispersion value of a gemstone.
While we indicate the yellow (sodium) light with the capital letter D, the red light is indicated with the letter B and the blue light with the captical G.
You may read phrases as "the B-G interval" when there is a reference to dispersion. It merely means that they talk about the interval between nB and nG. In otherwords, the difference between the refractive index measured with red light and the refractive index measured with blue light.